ext/tk/extconf.rb: fix build error with tk variant

* ext/tk/extconf.rb: use -rpath linker option instead of -R, to
  fix build error with tk variant.  incorporate from MacPorts.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44498 b2dd03c8-39d4-4d8f-98ff-823fe69b080e